1. Field of the Invention
The present invention relates to a spray gun, particularly though not exclusively for spraying paint.
2. Description of the Related Art
In this specification the term “spray liquid” is used to refer to paint or other liquid to be sprayed.
Generally spray guns are of two types, (i.) those operated by pumping the spray liquid to a nozzle whence it sprays under momentum imparted by being pumped through the nozzle and (ii.) those operated by blowing gas—usually air—past a nozzle, the air drawing the spray liquid from the nozzle and imparting momentum to it. Flow of paint can be regulated with a needle extending into the nozzle. The spray liquid may enter the nozzle at atmospheric or elevated pressure. This type of spray gun is referred to here as an “air spray gun”. Air spray guns themselves are of two further types, those operating at conventional compressed air pressure and those operating at low pressure, but with high volume flow. These are known as HVLP spray guns. The invention relates to BOTH types of air spray gun.
Paints are of widely differing viscosities. Thus some thick paints require one needle and other thin paints require another needle. Needle exchange is awkward, to the extent that in European Patent Application No. 1,340,550, in the name of J Wagner GmbH, interchange of the complete front of the gun is proposed.
In our existing air spray gun, which is an HVLP gun, the needle is exchanged by removal of the needle backwards, leaving the nozzle in place. Simultaneously the nozzle can be removed forwards, by unscrewing, once the air cap is removed.
Typically an air spray gun comprises:                an air flow passage in the gun;        a spray liquid nozzle mounted in the air flow passage and having an orifice for spray liquid to flow from, the orifice being on the downstream side of the spray liquid nozzle with respect to both air and spray liquid flow;        a needle extending from upstream into the spray liquid nozzle for regulating its orifice and flow of the spray liquid;        a spray liquid flow passage to the spray liquid nozzle, with the needle extending in the spray liquid flow passage;        a flow-passage/needle seal at a position along the needle spaced from the spray liquid nozzle;        a trigger mechanism connected to the needle to regulate its position;        an air cap defining with the spray liquid nozzle:                    a convergent air flow region immediately upstream of the spray liquid orifice,            a central opening from which spray liquid entraining air flows and            a pair of opposed horns having spray patterning bores with orifices directed inwards for patterning the spray liquid containing air flow from the central opening and                        an air distributor upstream of the air cap for distributing air to the patterning bores.        
This air spray gun is referred to as being “of the type defined.”